Directional microwave antenna



Oct. 4, 1949. c, c, UTLER 2,483,575

DIRECTIONAL MICROWAVE ANTENNA Filed July 26, 1944 6 Sheets-Sheet 1 FIG.

TRANSLA T/ON DEV/CE POINT BEAM ANTENNA FIG. 2 I76. 3

REAR wsw or LONG HEAD 12 ELEVA T/ONAL wzw or LONG HEAD l2 PLAN SECTIONALVIEW or LONG HEAD l2 2 INVEN TOR c. c. CUTLER A TTORNEV Oct. 4, 1949. c.c. CUTLER DIRECTIONAL MICROWAVE ANTENNA 6 Sheets-Sheet 3 Filed July 26,1944 A 7' TORNE) Oct. 4, 1949., c. c. CUTLER 2,433,575

I DIRECTIONAL MICROWAVE ANTENNA Filed July 26, 1944 e Sheets-Sheet 5FIG. /7 49 TRANSLA T/ON DE VICE E- PLANE k H-PLAIVE A T TORNE V t. 4,1949. c. c. CUTLER I 2,483,575

DIRECTIONAL MI CROWAVE ANTENNA Filed July 26, 1944 6 Sheets-Sheet 6DIRECTIVE CHARACTER/ST/C FOR MODIFIED LONG HEAD /2 OF INVENTION (FIG.2/)

F/G. Z3

ly-PLANE E-PLAN E 73 64 r! l -25" I I I v 40-- I I I I A TTORNEV 'larapertures or slots facing the reflector.

Patented Oct. 4, I949 DIRECTIONAL MICROWAVE ANTENNA Cassius C. Cutler,Oalghurst, N. 3., assignor to Bell Telephone Laboratories, Incorporated,New York, N. Y., a corporation of New York Application July 26, 1944,Serial No. 546,687

This invention relates to antenna systems and particularly to microwavedirective antennas.

Patent 2,422,184 granted on June 17, 1947, to applicant discloses andclaims a highly directive antenna system comprising a paraboloidalreflector hving a circular periphery, a rectangular wave guide extendingthrough the reflector vertex and along the reflector axis to a pointslightly beyond the reflector focus, and a head or resonant chamberconnected to the end of the wave uide and having a pair of spacedrectangu- The long transverse dimensions or the apertures and guide areparallel, the long transverse dimension of each aperture being in theorder of 0.59 to 1.0 wavelength and smaller than that of the guide. Intransmission, each aperture in efiect energizes or illuminates adifferent half of the reflector. The wave front produced by the primaryantenna comprising the two apertures is nearly spherical, that is, theso-called phasing characteristic in the plane of electric polarizationand the phasing characteristic in the plane of magnetic polarization arefairly flat. The major lobe patterns of the primary antenna aresufliciently wide to illuminate properly the reflector and aresubstantially the same. The axis of the major lobe pattern in each planeof the complete rear-feed antenna system is aligned with the reflectoraxis and the half power widths of the major lobe patterns of the systemin the two planes are relatively small and also substantially the same.Stated difierently, the system has a poin beam. Ordinarily, in azimuthalscanning airborne radar systems utilizing the above described antenna,the electric and magnetic planes of polarization are horizontal andvertical, respectively.

While the point-beam antenna described above is highly satisfactory andhas been successfully used, it has been found in practice that theso-called quadrature minor lobes extending at right angles to thereflector axis are often quite pronounced. Hence when the antenna isused in an airborne azimuthal scanning radar system, stron echo signalsmay be received from the earths surface beneath the aircraft, wherebyambiguous target indications are "obtained. Accordingly, it appearsdesirable to improve the point-beam antenna system described above forthe purpose of reducing or eliminating the so-called quadrature minorlobes. Also, it appears advantageous to use, in a point-beam systemcomprising a paraboloidal reflector having a circular periphery, aprimary antenna or head having a very wide band characteristic and morenearly flat phasing characteristic.

Inaddition, for certain radar uses, a ,fanbeam antenna, that is, onehaving adirective 7 Claims. (Q1: 25033.63)

2 characteristic in which the half power widths of the major lobepatterns in the electric and magnetic planes are substantiallydiflerent, is preferred over a point-beam antenna. In accordance withthe present invention, a paraboloidal reflector having an elliptoid orquasielliptical periphery, is employed to secure a fanbeam and theimproved primary antenna or head mentioned above, slightly modified asexplained below or not, is utilized to secure optimum energization orillumination of the undulous elliptoid reflector.

As used herein the term feed generically applies to a transmitting orreceiving primary antenna used in front of a reflector such as aparaboloidal reflector. Also the terms horizontal fan-beam and verticalfan-beamf refer to fan-beams wherein the major lobe patterns of greaterwidth are included, respectively, in the horizontal plane and thevertical plane. The term aperture is used in its physical or mechanicalsense and not in its optical or electrooptical sense; that is, as usedherein, it signifies a passage or a hole, and not the width or diametraldimension taken at the periphery of the paraboloidal reflector.

It is an object of this invention to obtain highly unidirectionalradiant action.

It is another object of this invention to reduce or eliminate, in anairborne radar system, echo signals from the earths surface beneath theaircraft.

It is another object of this invention to obtain, in a point-beamantenna system, narrow major lobe patterns in the electric and magneticplanes and negligible minor lobes, particularly negligible quadratureminor lobes, in said planes.

It is another object of this invention to obtain, for use with aparaboloidal reflector, a feed or primary antenna which emits orreceives a spherical wave front.

It is another object of this invention to obtain a highly eflicient,high gain fan-beam antenna system.

It is another object of this invention to energize a paraboloidalreflector having a non-cir- .cular periphery in an optimum manner andwithout substantial energy loss.

In accordance with one embodiment of the invention, in a point-beamantenna system, such as the prior art system disclosed in my patentmentioned above, the long transverse dimension of each of the tworectangular apertures is in the order of 1.28 wavelength and thereforeconsiderably greater than the corresponding aperture dimension of theabove-described prior art head. Also, by way of com-' parison', the longaperture dimension is greater 3 than the long transverse dimension ofthe guide. As compared, respectively, to the major lobe magnetic andelectric plane patterns for the point-beam head included in the priorartsystem and hereinafter termed the short head, the major'lobe magneticplane pattern is considerably more narrow, and the major lobe electricplane pattern is to a less extent more narrow. In other words, the headincluded in the system of the present invention hereinafter termed thelong head, has a horizontal fan-beam. As a result, the illumination orenergization of the central or vertex portion of the reflector and theflector are greater and smaller respectively, than those produced by theprior art head; and the quadrature minor lobes, which are establishedprimarily by the peripheral portion of the reflector, are relativelysmall. Consequently, .the echo signals from the earths surface beneaththe aircraft are minimized. At the same time, the half power widths ofthe major lobe patterns in the electric and magnetic planes are notchanged materially.

In accordance with another embodiment, a rear-fed paraboloidal reflectorhaving an elliptoid periphery and symmetrically disposed relative to thereflector axis, is associated with a dual-aperture head or primaryantenna. More particularly, the projection of the reflector periphery ona plane perpendicular to the reflector axis, such as the planecontaining the latus rectum of the reflector, is an ellipse; and theactual curvature, considered in the solid or in three dimensions, of theperiphery is undulous and almost elliptical, that is, quasi-ellipticalor elliptoid. The major axis and the minor axis of the elliptoidperiphery are substantially horizontal and vertical, respectively,whereby a fan-beam having a small half power width in the horizontalplane and amuch larger width in the vertical plane, that is a verticalfan-beam, is secured. While the dual aperture head may be of the priorart short type or of the long type described above, the long type ispreferred inasmuch as the horizontal fanbeam of the long head produces,as is desired, a sharply tapered or optimum illumination of thequasi-elliptical reflector in the magnetic plane and, in the electricplane, a less sharply graded illumination which is optimum for the widerreflector. In this connection it will be noted that the fan-beamdirective characteristics, considered in the solid, of the long head andof the quasi-'- elliptical reflector, are the inverse of each other. Ina modification, the optimum illumination taper in the electric plane,for a quasi-elliptical reflector, is secured by attaching a verticalWedge or triangular reflective member to the top wall of the guide and asimilar vertical member to the bottom guide wall. By reason of the Widevertical reflecting surface formed by the wide wall of the guide and thetwo wedge members, the right and left halves of the quasi-ellipticalreflector are each energized by only one of the dual slots,substantially. In more detail, each reflector half is illuminated bywavelets propagated directly from the associated rectangular apertureand, in addition,'the illumination of the peripheral reflector portionfarthest removed from the slot is enhanced by wavelets propagatedindirectly via the wide reflecting surface, or image slot, wherebyoptimum illumination of the quasi-elliptical reflector obtains.

The invention will be more fully understood energization of theperipheral portion of the refrom a perusal of the followingspecification taken in conjunction with the drawings on which likereference characters denote elements of similar function and on which:

Fig. 1 is a perspective view of a point-beam antenna system constructedin accordance with the invention; and Figs. 2, 3 and 4 are respectivelyrear, elevational and sectional plan views of the long head included inthe embodiment of Fig. 1;

Fig. 5 illustrates the magnetic or H-plane and the electric. or E-planedirective curves for the longheadused in the system of Fig. 1 and for aprior art short head such as disclosed in my patent mentioned above;

Fig. 6 is a curve showing the relation between the width of the H-planemajor lobe for the long head and the length of each aperture in thehead; and Figs. 7 and 8 illustrate respectively, the phasingcharacteristic and the bandwidth characteristic of the long head of Fig.1;

Fig. 9 illustrates the I-I-plane and the E-plane directivecharacteristics taken for a scanning sector centered on the reflectoraxis of the complete system of Fig. 1; and Fig. 10 illustrates thecompanion curves for a prior system comprising the prior art short head;

Figs. l1, l3 and 15 illustrate the directive characteristics, taken fora sector centered on a line perpendicular to the reflector axis, of thecomplete system of Fig. l; and Figs. 12,14 and 16 illustrate thecorresponding companion curves for the aforementioned prior art system;

Fig. 17 is a perspective view of a fan-beam antenna system constructedin accordance with the invention; and Fig. 18 is a front view of thereflector included in the system of Fig. 17

Figs. 19 and 20 are directive curvees for a system constructed inaccordance with Fig. 17 and comprising, respectively, a medium sizereflector and a large reflector;

Fig. 21 is a perspective view of a fan-beam antenna system constructedin accordance with the invention and comprising a modified long head orprimary antenna;

Fig. 22 illustrates the directive characteristic for the modified longhead of Fig. 21;' and Fig. 23 illustrates the directive characteristicfor the complete system of Fig. 21.

Referring to Figs. 1, 2, 3 and 4 reference numeral 1 denotes aparaboloidal reflector having a substantially circular periphery 2, avertex 3, an

axis 4 and a focal point 5. Numeral 6 denotes a translation device whichmay be a transmitter, a receiver or a radar transmitter, and numeral 1designates a rectangular air-filled wave guide which passes through thevertex 3 of reflector. l and extends along the axis 4 to a pointslightly beyond the focus 5. The guide I comprises a main section 8which is connected to device 6, an end section 9 having an end opening i0 and a tapered impedance-matching section I l connecting sections 8 and9. Reference numeral l2 denotes a dual aperture long head horn whichencloses the open end of guide section 9 and comprises the three brassplates l3, l4 and IS, the rubber gasket [6, the dielectric plate l1 andthe brass plate l8, all held securely by brass screws l9. Numeral 28,Fig. 4, designates a resonant chamber or-cavity in plate l4 and numerals2| and 22 denote two antenna apertures which extend through the platesI5, i6 and I8. Portions 23 and 24 of the dielectric plate ll extendacross the antenna apertures 21 and 22, respectively, and constitutedielectric windows. Numeral 25 denotes a threaded plug for tuning thecavity 20.

Each of apertures 2| and 22 has a long transverse dimension La which isgreater than the lon transverse dimension Lg of guide section 9 and inone specific embodiment is in the order of 1.62 inches, as indicated inFig. 6, corresponding to 4.12 centimeters and 1.285 wavelengths at themean or design operating wavelength of 3.2 centimeters. The width We. ofeach of the apertures in the aforementioned specific embodiment is inthe order of 0.1 wavelength and the spacing S between the centers of theapertures is in the order of 0.5 wavelength. Also the length Ln andwidth Wh of head i2 in the specific embodiment are, respectively, 2 and1.5 inches. For comparison the length 111:, representing the longtransverse dimension of the apertures in the prior art short head, isshown.

In operation, assuming device 6 is a transmitter or transceiver,microwaves supplied by device 6 are conveyed by guide to the resonantcavity 20 and the antenna apertures 2| and 22, as indicated by thearrows 26, Fig. 4, and wavelets are emitted by the apertures. Forpurpose of explanation, it is assumed that the waves are horizontallypolarized, that is, the E-plane and H- plane are horizontal andvertical, respectively. The wavelets impinge upon reflector l and arereflected or redirected along the general direction of the reflectoraxis 4. In reception, the converse operation obtains by reason of thereciprocity theorem.

In more detail, and as explained below in connection with Figs. 5, 6, 7and 8, the wavelets emitted by the apertures are cophasal and thesewavelets combine to produce a nearly spherical wave front having, byvirtue of the critically selected aperture length La, a sharply taperedintensity variation, whereby the taper of the reflector illumination isrelatively great or sharp as compared to the illumination taper realizedin the prior are system of my copending application. As explained in myabove-mentioned patent, a highly satisfactory over-all directivecharacteristic, taken over a scanning sector of say, 40 degrees, isobtained when the illumination of the reflector decreases uniformly froma maximum at the vertex to a value of about ten decibels be.- low themaximum at the reflector mriphery. In other words, the efiective portionof the E-plane or H-plane major lobe pattern for that head may beconsidered to be the central portion which is centered on the lobe axisand extends between the plus and minus directions or angles havingintensities ten decibels below the maximum, approximately.

Referring to Figs. 5 and 6, reference numerals 21 and 23 designaterespectively the I-I-plane and E-plane major lobe patterns for the longhead I 2 of Fig. 1; and numerals 29 and 30 denote the H- plane andE-plane patterns for the prior art short head. In Fig. 6, the curve 3|illustrates the relation between the H-plane dimension or length La ofapertures 2| and 22 and the effective angle of illumination, that is,the width taken at a point ten decibels below the peak of the H-planemajor lobe pattern for the long head l2. In Fig. 5, the Width of theH-plane lobe 21 for the long head l2 at the ten-decibel point,represented by line 32, is considerably less than the correspondingwidth of the I-I-plane lobe 29 for the prior art short head. In theH-plane, the illumination produced by the long head I2 is effectiveprimarily over the central or vertex portion of the reflector or, moreaccurately, over the bl-degree sector included between the plus37-degree and Cir .6 theminus 37-degree directions; and the illumination of the peripheral portions of the reflector and at angles beyondthe periphery is negligible whereby, as discussed below in connectionwith Figs. 11 and 12, the quadrature minor lobes in the vertical planeare of relatively low intensity and ground reflection is reduced. Asindicated by point 33 on curve 3|, Fig. 6, the optimum lengthLe-corresponding to the optimum major lobe 21, Fig. 5, having at theten-decibel point 32 an eifective illumination angle of '74 degrees is1.62 inches. Considering the E-plane, Fig. 5, the Width at point 32 ofthe lobe 28 for the head I2 is less than the corresponding width of theE-plane lobe 30 for the short head, but somewhat greater than theeffective width of the H-plane lobe 2! for the long head l2; Since theE-plane is horizontal, ground reflections do not occur in this plane.Hence, the somewhat larger angle of illumination produced by head l2 inthe E-plane, as compared to that produced in the I-I-plane, is notdetrimental in an airborne azimuthal scanning radar.

Referring to Fig. 7, the vertical scale represents the phase angle indegrees of the wavelets arriving at the circumference of a referencecircle having its center at the focus or head and included in theI-I-plane or E-plane, zero phase being on the circumference of thereference circle. The horizontal scale represents angular directions asmeasured in degrees, the zero direction being coincident with thereflector axis 4. Reference numeral 34 denotes the ideal flat phasingcharacteristic, and numerals 35 and 36 designate, respectively, theI-I-plane and E-plane phasing characteristics for the head I2. Numerals3i and 38 denote the I-I-plane and the E-plane phasing curves for theprior art short head. It will be noted that the I-I-plane characteristicfor the long head almost coincides with the ideal characteristic 34 andis greatly superior to the H- plane curve 31 for the short head. Stateddifferently, in the H-plane, the wave front produced by the long head I2is, as is desired, circular; or stated still differently, the waveletsarriving at the circumference of the reference circle are almost incomplete phase agreement. Also, the E plane characteristic 3B isrelatively flat and su perior to the E-plane characteristic 38 for theshort head.

- Referring to Fig. 8, reference numeral 39 denotes the measuredstanding wave band-width characteristic for the long head i2 of Fig. 1.As shown, the impedance of the head l2 and guide section 9 issubstantially matched at wavelengths included in the band extending from3.14 to 3.26 centimeters. For a normal radar Wavelength band, centeredon a design wavelength of 3.20 centimeters and extending from 3.17 to3.23 centimeters, the characteristic is relatively flat.

Referring to Figs. 9 and 10, reference numerals 40 and 4|, Fig. 9,designate respectively, the measured I-I-plane and E-plane partialdirective patterns for a system constructed in accordance with Fig. land comprising a reflector having a substantially circular opening, thelong and short dimensions of the opening bein 13% and 14% inchesrespectively, and the focal length being 6.3 inches. The patterns weretaken for the central 40 (:20) degree sector; Numerals 42 and 43, Fig.10, denote respectively, the measured H-plane and E-plane partialdirective patterns for the prior art system comprising a short head. Ineach pattern numeral 44 denotes the major lobe, numerals 45 denote'the'two minor lo'bes adjacent the major lobe 44', andnumerals 46 denote thenulls between the major lobe 44 and the, adjacentminor lobes 4B. Thehalf power widths, taken at a point, 41 three decibels down from thepeak lobe value of the I-I-plane and E-plane major lobes 44 of patterns48 and M, Fig. 9, are substantially the same, the Widths being 6.6degrees and 5.2 degrees respectively. In a comparable embodiment of theprior art system, the H-plane and E-plane half power major lobes areeachexactly 6.2 degrees. Hence the system of Fig. 1 produces a pointbeam which is onlyslightly diiferent from the point beam of the priorart system. As is desired, the

' H-plane minor lobes, 45, Fig. 9, are of less incompanion E-planedirective pattern 43, Fig. 14,

correspond respectively to the H-plane and E- plane patterns, 42 and 43,Fig, 10, for the system of the prior art. In the patterns of Figs. 11,12, 13 and 14, numerals 48 denote the quadrature minor lobes, theselobes being included in the 80 to 120-degree sector centered on the90-degree direction. Generally considered, the H-plane quadrature minorlobes 48, 11, for the embodiment of the invention illustrated by Fig. 1

are, as is desired, more than 45 decibels below the major lobe peak,whereas in the prior art system the corresponding lobes 48, Fig. 12, areonly about 30 decibels down. Hence, as already discussed, the long headl2 functions as compared to the prior art short head to suppress oreliminate more completely the undesired ground echoes. In addition, theE-plane quadrature minor lobes 48, Fig. 13, for the system of Fig. 1 areof less intensity than the corresponding quadrature lobes. 48, Fig. 14,for the prior art system.

Fig. 15 illustrates a measured partial or quadrature E-plane pattern forthe system of Fig. 1, the pattern being centered on the H-planel. Fig.16 illustrates the companion partial E-plane pattern for the prior artsystem. These patterns were taken with the reflector axis 4, Fig. 1,vertical, for the purpose of avoiding ground effects; and they aretherefore more truly indicative of the action in the E-plane than thecurves of Figs. 13 and 14. As shown by the curves of Figs. 15 and 16,the E-plane quadrature minor lobes 4B, Fig.15, for'the system of. Fig.1, are of considerable less intensity, as is desired, thanthecorrespondingquadrature lobes 48, Fig. 16, for the prior art system.

Referring to Figs. 17 and 18, reference numeral 49 denotes aparaboloidal reflector having an undulous elliptoid or quasi-ellipticalperiphery se, the horizontal major axis and the vertical minor axis ofthe periphery being denoted, respectively, by the reference numerals 5iand 52. The axes 5i and 52am each parallel to the latus rectum planewhich plane, as in all conventional parabolic reflectors, contains thefocal point 5 and extends perpendicular to the axis 4. The

7 remaining portion or the system comprising the device 6, guide Iand'head I2 is the same as the system of Fig. 1. In operation, thequasi-elliptical reflector 49 is energized in an optimum manner, sincethe head l2 has a horizontal fanbeam. Note that the effective width atpoint 32, Fig. 5, of the E-plane lobe 28 for the long head is greaterthan the effective width of the H- plane lobe 21 for this head. Thequasi-elliptical reflector, taken alone, has a vertical fan-beamcharacteristic since the reflected beam is more sharply focussed in thehorizontal plane than in the vertical plane.

Referring to Figs. 19 and 20, numerals 53 and 54 denote respectively,the measured H-plane and E-plane patterns for two systems constructed inaccordance with Fig, 17 and having reflectors of different size. In thesystem corresponding to Fig. 19 the paraboloidal reflector has a focallength of 8 inches; and the major and minor axes of the quasi-ellipticalperiphery are 21 and 12 inches respectively; In the system correspondingto Fig. 20 the reflector has a focal length of 10.5 inches and the majorand minor axes of the quasi-elliptical periphery are 29 and 16 inches,respectively. For optimum operation with the head l2 the ratio of themajor and minor axes of the periphery should be in the order of 2.0; andthe ratio of the major axis of the quasi-elliptical periphery to thefocal length should be in the neighborhood of 2.6.

As shown by the curves of Figs. 19 and 20 the system of Fig. 1'7produces a fan-beam. In Fig. 19, the I-I-plane major lobe pattern has ahalfpower width, taken at line 41, of 6.6 degrees and the E-plane majorlobe pattern has a half-power width of 4.2 degrees. The fan-beam of Fig.20 is more pronounced, the H-plane and E-plane half-power widths of themajor lobe patterns being respectively 5.2 and 3.0 degrees. Since, in afan-beam radar, the target direction is usually determined only in theplane of the sharper or more narrow major lobe pattern, that is, theE-plane, the E-plane minor lobes 45, Figs. 19 and 20, are highlysatisfactory, since they are small, and the somewhat larger I-I-planeminor lobes 45 and relatively shallow I-I-plane nulls 46 are notespecially detrimental. Note that the first E-plane minor lobes 45, Fig.20, adjacent the major lobe 44, are extremely low. In addition, the gainof the'system of Fig. 17 over a standard comparison antenna, as measuredon the common axis 4 of the reflector and the major lobe, is very high.Specifically, the gain of the system comprising the smaller reflector,Fig. 19, is 30.2 decibels and the gain of the system using the largerreflector, Fig.20, is 32.9 decibels.

Referring to Fig. 21, the system is the same as that illustrated by Fig.17, except that the head I2 contains a slot 55 and triangular reflectivemembers or wedges 5t and 57 positioned in slot 55 and attached,respectively, to the top and bottom walls of guide section 9.Considering either aperture, 25 or 22, the vertical wall of guide 9 andthe wedges 55 and 51 constitute a vertical reflective surface 58 whichfunctions to direct the wavelets proceeding from the aperture towardsthe outermost peripheral portion of the left or right half of thereflector, whereby as explained below optimum illumination of thereflector 48 is obtained. In other words, each right or left half of thereflector sees a different vertical real slot and, in the verticalreflecting surface, the image of the real slot. The surface 58 also in asense shields the two apertures 2i and 22 from each other.

Referring to Fig. 22, reference numerals 59 and B denote, respectively,the H-plane and E-plane major lobe patterns for the wedge or modifiedhead I2 of Fig. 21. The Widths of the H-plane lobe El and the E-planelobe 62, taken at the effective illumination point 32, ten decibelsdown, are greater respectively than the corresponding widths, Fig. 5,for the long head without the Wedges. Thus the I-I-plane and E-planeeffective lobe widths, Fig. 22, for the modified head are, respectively,83 and 164 degrees, whereas the H- plane and E-plane effective widthsfor the head of Fig. 5 are 73 and 130 degrees, respectively. By reasonof the increase in the effective lobe widths, especially in the E-plane,the illumination is suitable for a quasi-elliptical reflector of greaterdepth, as measured by the ratio of the major axis dimension to the focallength. In certain installations it is advantageous to use a focallength which is as short as practicable; and because the E-plane majorlobe is wider for the modified head of Fig. 21, than for the unmodifiedhead of Fig.

and 64 are below 22 decibels and are, therefore,

negligible.

Although the invention has been described in connection with certainspecific embodiments, it is not to be limited to the embodimentsdescribed inasmuch as other apparatus may be employed in successfullypracticing the invention.

What is claimed is:

1. A concave antenna reflector having a varying peripheral diameterextending perpendicular to its axis, the projection of the reflectorperiphery on the latus rectum plane of the reflector being an ellipse,the maximum axial depth of the reflector being less than its focallength.

2. In a directive radio system, means for transmitting an approximatelyspherical wave front comprising a rectangular wave guide, a translationdevice connected thereto, said guide comprising a first metallic wallhaving a pair of antenna apertures spaced in a given plane for emittingor collecting wave components, a second metallic wall connected to saidfirst wall between said apertures and shielding said apertures from eachother, the long transverse dimension of each aperture being greater than1.25 wave lengths and the short transverse dimension of each aperturebeing 0.1 wavelength.

3. A system in accordance with claim 2, the long transverse dimension ofeach aperture being 1.62 wavelengths and the short transverse dimensionof each aperture being 0.098 wavelength.

4. In an antenna system, a secondary passive member having an axis andan elliptoid periphery symmetrically disposed relative to said axis, aprimary member facing said secondary member and having a pair ofrectangular apertures, the long dimension of each aperture being greaterthan 1.25 wave lengths, the longest diameter of said elliptoid peripherybeing perpendicular to said axis and to the long dimensions of saidapertures, and a translation device connected to the primary antenna.

5. In an antenna system, a concave reflector having a focus, arectangular wave guide connected to a translation device, said guideextending through said reflector and having an end opening near saidfocus, a Wave guide chamber or cavity comprising a first metallic wallfor receiving the end of said guide and a second metallic wall facingsaid opening, said first wall being included between said reflector andsaid opening and having a rectangular aperture facing said reflector,the long transverse dimensions of said guide and aperture being paralleland the long transverse dimension of said aperture being greater than1.25 wave lengths and greater than the long transverse dimension of saidguide.

6. In combination, a reflector having a concave reflecting surface andan elliptoid periphery, a focus and a principal axis, a rectangular waveguide extending through said reflector and along said axis, said guidehaving an end rectangular opening positioned substantially at saidfocus, a resonant chamber comprising a first metallic wall for receivingthe end portion of said guide and a second metallic wall facing saidopening and said first wall, said first wall being positioned betweensaid opening and said reflector and having a pair. of spaced rectangularapertures facing different portions of said concave reflector, saidguide being included between said apertures, the corresponding sides ofsaid apertures and said opening being substantially parallel and thelong side of said apertures each being greater than 1.25 Wave lengthsand greater than the long side of said opening, and a translation deviceconnected to said wave guide.

7. In combination, a concave reflector having an axis and a focus, aresonant Wave guide chamber positioned at or near said focus andcomprising a metallic wall, said Wall having a pair of rectangularapertures facing said reflector and positioned in the latus rectum planeof said reflector, a rectangular Wave guide extending along thereflector axis and connected to said Wall between said apertures, thelong transverse dimensions of said apertures being greater than 1.25wave lengths and greater than the long transverse dimension of saidguide, and a pair of reflective shield members attached to said guideand Wall and positioned between said apertures, whereby each apertureenergizes a different half of said reflector substantially.

CASSIUS C. CUTLER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,206,923 Southworth July 9, 19402,232,559 Rice Feb. 18, 1941 2,283,935 King May 26, 1942 2,342,721Boerner Feb. 29, 1944 2,370,053 Lindenblad Feb. 20, 1945 2,409,183 BeckOct. 15, 1946 2,422,184 Cutler June 17, 1947 2,434,253 Beck Jan. 13,1948 2,441,574 Jaynes May 18, 1948 FOREIGN PATENTS Number Country Date402,834 Great Britain Dec. 14. 1933

